The present invention relates to a circuit arrangement for a voice-controlled hands-free facility connectable via a hybrid circuit to a subscriber facility comprising a microphone, an internal and an external loudspeaker, and two series combinations of an input amplifier, a variable attenuator, and an output amplifier, one for the send path and one for the receive path, with the output of the input amplifier and the output of the variable attenuator connected to a voice control unit coupled to the variable attenuator by a line.
Voice-controlled hands-free facilities (cf., inter alia, M. Slawik and H. Wiedmann: "Die neue sprachgesteuerte Freisprecheinrichtung fur die Produktfamilie INTERMAT", Elektrisches Nachrichtenwesen, Vol. 53, No. 4, 1978) give the user greater freedom of movement during a telephone conversation because, instead of the transmitter and receiver capsules of the handset, a microphone and a separate loudspeaker are provided.
However, hands-free facilities have the disadvantage that a signal reproduced by the loudspeaker follows the air path and is received by the microphone, and that a coupling path exists between the microphone and the loudspeaker in the form of sidetone path of the hybrid, so that a closed loop is formed; as a result, a singing tone may be produced due to feedback.
If the loudspeaker and the microphone are accommodated in the housing of the telephone set, i.e., if the hands-free facility is a single-part facility, the short distance between the mircophone and the loudspeaker results in a low air-path attenuation. An increased voice-controlled swing is thus required to avoid feedback. This hard voice-control behavior has the disadvantage that, in the presence of loud ambient noise, one path may be permanently switched on, whereby speech contact is broken. Furthermore, duplex operation, which is usual in normal telephone communication, is no longer possible because the associated transmit path is not switched on until the voice has reached a certain level.
If, instead of the internal loudspeaker, an external loudspeaker is connected to the telephone set, the considerably higher air-path attenuation cannot be turned to account because of the necessary hard voice control for operation with the internal loudspeaker.